Steel workpieces, as typified by gears, are tested after being manufactured. In this test, only steel workpieces that have passed predetermined acceptability criteria can be advanced to the next step. Nondestructive testing is required in such cases. Such nondestructive testing does not compromise the shapes of the steel workpieces and allows all the workpieces to be tested.
Various apparatuses have been proposed for performing nondestructive testing, one of which is an eddy-current testing apparatus that uses eddy currents, as shown in Japanese Patent Application Laid-Open Publication No. 2004-108873, for example. This eddy-current testing apparatus is described with reference to FIG. 15 hereof.
An excitation coil 102 and a detection coil 103 are wound around a cylindrical steel workpiece 101, as shown in FIG. 15. AC voltage is applied to the excitation coil 102 from an AC power source 104. This causes an eddy current to be generated in the surface layer of the steel workpiece 101. An alternating current is generated in the detection coil 103 by this eddy current. The voltage of this generated alternating current is measured by a measurement device 105. The strength and carburized depth are determined based on this detected voltage.
Voltage is applied to the steel workpiece 101, and since the generated voltage is merely detected based on the eddy current, there is no danger of the steel workpiece 101 being scratched, and the quality can be tested without damaging the workpiece. Therefore, complete testing of the steel workpiece 101 is possible.
Because of the shapes of the excitation coil 102 and detection coil 103, the shape of the steel workpiece 101 is limited to a shaft, a cylinder, or a tube, and application is difficult if the steel workpiece 101 is a flat plate. The application of the eddy-current testing apparatus is thereby limited.
In view of this, there is a need for an eddy-current nondestructive testing apparatus that can be used when the steel workpiece 101 is shaped either as a cylinder or as a flat plate.
The distance L1 from a top land 108L on the left side of a gear 107 to the detection coil 103 should preferably be constant, as shown in FIG. 16. This is because when the distance L1 changes, the detected voltage fluctuates, and measurement precision is reduced. With a manual testing apparatus in which the distance L1 is determined manually, it is difficult to keep the distance L1 constant. With an automatic testing apparatus in which the distance L1 is automatically determined by a robot or the like, the testing apparatus is expensive and large.
In view of this, there is a need for a small and inexpensive nondestructive testing apparatus in which the distance between the tested object and the detection coil can be kept constant.
Furthermore, when the voltage based on the eddy current is detected by a detection coil 103 using the gear 107 as the testing object, the entire external periphery of the gear 107 acts as the testing object for this detected voltage. In other words, information on the top land 111 of the gear 107 and information on the bottom land 112 are combined and harmonized. It is common for defects to readily occur in the bottom land of the gear 107, but when combined information is used, there is a danger that if there are defects in the bottom land 112, these defects will not be detected.
In view of this, there is a need for a nondestructive testing apparatus in which the bottom land 112 alone of the gear 107 can be tested.